The principle of electromagnetic force compensation has a wide field of applications in the most diverse kinds of weighing scales and balances that are employed in commerce, in industry, and in laboratories. This principle has the particular strength that weighing instruments of enormous measurement accuracy can be realized with it. For example, an analytical balance functioning according to the principle of electromagnetic force compensation offers the capability to measure a weighing load of 100 g with a measurement resolution of 0.01 mg, i.e. with a precision of one part in ten million.
A gravimetric force-measuring device of the generic type to which the present invention relates has a stationary base part, a load receiver constrained to the base part for guided mobility and serving to receive the weight force of a weighing load, a permanent magnet assembly with an air gap fastened to the base part, a coil that is movable in the air gap and carries a compensation current, as well as a force-transmitting mechanism that connects the load receiver to the coil. An optoelectronic position sensor, whose sensor signal corresponds to the amount of deflection by which the interconnected moving parts of the balance mechanism are displaced out of a null position as a result of placing a load on the load receiver, includes typically a light emitter and a light receiver which are arranged on the base part facing each other across an interstice, as well as a shutter vane that cuts through the interstice and participates in the deflection of the moving parts.
The signal of the position sensor is sent to a controller which, in response, regulates the compensation current in such a way that the electromagnetic force between the coil and the permanent magnet returns the shutter vane and the connected movable parts of the balance to the null position. In other words, the function of the controller is to establish equilibrium between the electromagnetic compensation force and the weighing load. As the magnitude of the coil current and the force generated by it are proportionate to each other, the weight of a weighing load placed on the load receiver can be determined by measuring the coil current.
A balance operating in accordance with the principle of electromagnetic force compensation of the foregoing description is shown in DE 3 743 073 A1. The balance consists of a support console that stands in fixed connection to the balance housing and, by way of two swivel-jointed guide members, holds a load receiver with vertically guided mobility. On top, the load receiver carries the weighing pan which serves to receive the weighing object. A force corresponding to the mass of the weighing object is transmitted from the load receiver by way of a coupling element to the shorter lever arm of the reduction lever. The reduction lever is supported on the support console by means of two fulcrum flexures. A coil is connected to the longer lever arm of the reduction lever. The coil floats in the air gap of a permanent magnet system and generates the load-dependent counteracting force. The magnitude of the current flowing through the coil is regulated in the known manner by way of the optical position sensor and a servo amplifier in such a way that equilibrium is maintained between the weight of the weighing load and the electromagnetically generated counterforce. The optical sensor consists of the radiation emitter, the radiation receiver and the slotted aperture vane. The radiation emitter and the radiation receiver are fastened to the cover of the permanent magnet assembly, while the slotted aperture vane is fastened to the reduction lever. In this arrangement, the rearward end of the reduction lever, the coil and the slotted aperture vane according to the description of DE 3 743 073 A1 form a compact unit of high geometric stability.
A weighing system for a top-loading balance is shown in DE 103 26 699 B3. The weighing system includes a reduction lever split into two partial levers. The coupling element and the flexure fulcrum (in this case called bending fulcrum) of the reduction lever are likewise divided into two parts and are arranged laterally of the load receiver. The two partial levers are joined at the end, specifically at the longer lever arms, by a transverse connector which carries a slotted aperture vane and a coil mount for the attachment of the coil. This arrangement results in a very compact weighing system.
An optical position sensor is described in EP 2 607 866 A1. The main requirement that must be met by a position sensor of an electromagnetic compensation balance is the condition that the null position, i.e. the position of the shutter vane relative to the base part at which the transition of the sensor signal between negative and positive values takes place has to be maintained with the highest degree of accuracy and reproducibility. Furthermore, the sensor signal should be, as much as possible, a linear function of the deflection of the sensor vane. These requirements have to be satisfied in particular within a prescribed range of atmospheric temperature and humidity.
Within the subject area outlined above, the present invention is focused on the design of the force-transmitting mechanism and on how the optoelectronic sensor and the measurement transducer (for example a coil) are attached to the force-transmitting mechanism.
With the development of more and more powerful measurement transducers that are used in combination with force-transmitting mechanisms it becomes necessary to adapt the force-transmitting mechanisms to the increased forces which cause higher bending moments in the force-transmitting lever. If this factor is not considered in the design, the forces acting on the force-transmitting lever—on one side the weight force of the weighing load and on the other side the compensation force of the measurement transducer—can cause a bending deformation of the force-transmitting lever. Due to the inelastic behavior of the material of the force-transmitting lever, a downward displacement of the load receiver (i.e. a position shift in the direction of gravity) can occur over time, causing a load-related drift of the weighing result. This downward displacement of the load receiver further can further lead to an increase of the restoring forces of all flexure pivots (thin material connections), which can be viewed as springs that are characterized by an elastic spring rate and are subjected to a larger deflection as a consequence of the displacement. Over the long term, the increase of the restoring forces can likewise have an unknown effect on the measurement result, as the calculation parameter values that were stored in the processor unit for a specific operating point are no longer valid. These effects can be partially compensated through appropriate software measures. However, such a method of compensation relies on parameter values obtained from theoretical models and from experience and is not adequate for force-measuring devices of high precision.
In common state-of-the-art designs of force-measuring devices such as for example in WO 2014 169 981 A1, U.S. Pat. Nos. 4,938,301, 5,315,073, 4,245,711 or, as mentioned above, in DE 3 743 073 A1, the position-sensing function is performed by means of the shutter vane which is attached to the force-transmitting lever, i.e. the same lever to which the coil is attached. This has the advantage of simplicity of construction, as the same lever is being used. However, with the possible bending deformation of the force-transmitting lever that has been mentioned above, the position-sensing function is also compromised.
One possibility how the bending deformation could be counteracted is to improve the design of the force-transmitting mechanism in such a way that the force-transmitting lever will withstand the increased forces. However, the lever should on the one hand be as light as possible so that its mass inertia does not slow down the oscillatory return to the null position, but on the other hand the bending deformation of the lever should be minimized in view of the aforementioned inelastic behavior. Furthermore, the preferred production method is pressure die casting because of its low procurement- and manufacturing cost, but the inelastic properties of the finished parts are very unfavorable.
The coil is in most cases attached to the end of the longer lever arm of the force-transmitting lever. The corresponding permanent magnet core is arranged in the center of a cup-shaped cylindrical mantle whose function is to channel and to conduct the magnetic field as well as to contain the stray field and thereby to prevent interference with the adjacent electronic circuits, i.e. to shield the electronics from the magnetic field. Conversely, this shielding also reduces an unwanted influence of outside factors on the magnetic field of the permanent magnet. Openings in the mantle are necessary in order to allow the force-transmitting lever to enter the inside of the mantle, so that the coil can be held in the magnetic field. The openings in the mantle should be kept as small as possible, because otherwise the magnetic field could propagate to the outside of the mantle, or the force-measuring device could be adversely affected by parasitic extraneous fields. Any inhomogeneous zone in the system composed of the permanent magnet and the coil has a detrimental effect on the desired linearity of the force-measuring device. A switched arrangement in which the permanent magnet is attached to the force-transmitting lever and the coil is attached to the stationary leg of the parallel-motion guide mechanism is fraught with the same problems that have been discussed above.
The present invention therefore has the objective to provide a force-transmitting mechanism which is compatible with the installation of more powerful measurement transducers and which overcomes the drawbacks of the state of the art. In addition, the influence of extraneous fields acting on the force-measuring device from the outside should be minimized.